1. Technical Field of the Invention
This invention relates generally to providing electromagnetic interference (EMI) shielding in electrical equipment. More particularly, this invention relates to an electrical shield assembly operable to provide electromagnetic compliance (EMC) using a structural mold in telecommunications equipment racks.
2. Description of Related Art
The demand for telecommunications services has increased substantially due to the proliferation of local telephone lines, mobile subscribers, pagers, fax machines and the Internet. The high demand for telecommunications services translates into densely populated telecommunications equipment that must meet rigorous technical specifications and form factors. For example, Bellcore (now Telcordia) Specification GR-1089-CORE provides a set of generic criteria for EMC and electrical safety in network equipment. GR-78-CORE and GR-63-CORE provide, respectively, general physical design requirements and Network Equipment Building Systems (NEBS) physical protection criteria for the equipment.
Typically, the electrical requirements for the network equipment include compliance with various electrostatic discharge (ESD) tests as well as EMI susceptibility tests. Electrostatic charge may build up on any conductive surface of a telecommunications equipment rack such as, for example, a backplane disposed in a high speed fiber bank (HSFB), which can cause damage to electronic components therein. Therefore, the risk of ESD must be reduced or eliminated by establishing an effective discharge path to a ground source such as the equipment chassis.
Additionally, the electrical cabling disposed in a telecommunications equipment rack may emanate unwanted “electrical noise” that can cause EMI, which may affect electrical sub-systems that are not even connected to the cabling. As is well known, unmitigated EMI may lead to noise currents and intermittent data problems.
Several solutions addressing the ESD and/or EMI issues in telecommunications network equipment have been available for some time. Whereas such solutions are generally effective in satisfying the applicable standards, they are beset with numerous deficiencies and shortcomings, however. In general, the existing solutions can be divided into two types: EMI shields and EMI absorbing materials. EMI shields either reflect electromagnetic waves propagating towards the electronic equipment and prevent the waves from propagating into the shielded part of the electronic equipment or, alternatively, reflect electromagnetic waves generated from the inside of the electronic equipment inwardly such that they will not radiate out of the electronic equipment. Where the existing EMI shields are not of metallic composition, conductive platings or paints that limit EMI and ESD are typically used for encasing the electronic equipment, which is thereby provided with protection against disturbing electromagnetic waves. Typically, the EMI shields are deposited on the electronic equipment by either metal-arc spraying, vacuum metallizing, evaporating, cladding or electroplating.
In practice, however, it is not always easy to form EMI shields over the plastic surfaces of electronic equipment such as electrical cabling. Often, the plastic surfaces of electrical cabling have to be given a mechanical pretreatment in order to ensure positive depositing of the intended EMI shields. Moreover, mechanical pretreatments such as surface roughening may damage the electrical cabling.
The second type of existing solutions involve electric wave absorbing materials which convert incident electric waves to thermal energy so that the intensity of their transmission or reflection is reduced markedly. In typical implementations, the EMI absorption materials include ferrite cores which are rods comprised of metal and carbon blends. Ferrite cores are very effective in attenuating EMI when implemented in large numbers. Generally, eight or more ferrite cores, or ferrite cores having a rather larger size are required per electrical cable to effectively attenuate EMI. Although ferrite cores are effective in attenuating EMI when used in large numbers, they are heavy, bulky, expensive and brittle.
Other materials that are effective as absorbers of electric waves include compositions of aluminum, lead, zinc, titanium, lithium, steel, silver and copper. In particular, PZT (lead zirconate titanate) and PLZT (lead lanthanum zirconate titanate) are effective in absorbing electric waves at frequencies in the GHz band. However, the amount by which these compositions can be dispersed or incorporated into plastics is limited by various factors including the melt viscosity of the plastics used, their processability, mechanical strength, brittleness and adhesion of the fibers, films, sheets and other shaped parts of the plastics in which those metals are dispersed or incorporated. It is also known that if the incorporation of conductive metals is unduly small, satisfactory absorption characteristics are not attained.
Therefore, a need has arisen for a cost effective EMC solution that is capable of providing ESD prevention and EMI shielding to today's telecommunications equipment with relative ease, while still meeting the rigorous form factor requirements and technical specifications.